并行的foreach循环 - 奇怪的行为

Question

下面的代码只是创建一个List> of random numbers,然后计算并行foreach循环中每个列表的累积和.为什么我得到的数字少于'numLists'？通常在9990左右.我猜它与线程安全有关.什么是替代方法？(我是C#初学者,所以希望我使用正确的术语)谢谢.

using System;  
using System.Collections.Generic;  
using System.Threading.Tasks;  

namespace testParallelForeach
{
    class Program
    {
        static void Main(string[] args)
        {

            List<List<double>> bsData = new List<List<double>>();
            List<List<double>> cumsumDataP = new List<List<double>>();
            int numLists = 10000;
            int myLen = 400;
            Random rand = new Random();
            for (int i = 0; i < numLists; i++)
            {
                bsData.Add(new List<double>());
                for (int j = 0; j < myLen; j++)
                {
                    bsData[i].Add(rand.NextDouble());
                }
            }
            Parallel.ForEach(bsData, a => cumsumDataP.Add(CumulativeSumParallel(a)));
            Console.WriteLine("cumsumDataP.Count={0}", cumsumDataP.Count);
            Console.ReadKey();

        }

        public static List<double> CumulativeSumParallel(List<double> singleRetSeries)
        {
            int r = singleRetSeries.Count;
            List<double> cumsumList = new List<double>();

            cumsumList.Add(singleRetSeries[0]);
            for (int i = 1; i < r; i++)
            {
                cumsumList.Add(cumsumList[i - 1] + singleRetSeries[i]);
            }
            return cumsumList;
        }
    }
}

Answer 1

List<T>确实不是线程安全的,因此cumsupDataP.Add(...)以不可预测的方式丢弃数据.

将该行替换为:

ConcurrentBag<List<double>> cumsumDataP = new ConcurrentBag<List<double>>();

这一切都会奏效.请注意,这ConcurrentBag<T>是无序的,但这很好,因为无论如何你无法预测线程的顺序; p